Thyroid Hormone Receptor Alpha Is Required for Thyroid Hormone-Dependent Neural Cell Proliferation During Tadpole Metamorphosis.

Thyroid hormone (T-3) plays several key roles in development of the nervous system in vertebrates, controlling diverse processes such as neurogenesis, cell migration, apoptosis, differentiation, and maturation. In anuran amphibians, the hormone exerts its actions on the tadpole brain during metamorphosis, a developmental period dependent on T-3. Thyroid hormone regulates gene transcription by binding to two nuclear receptors, TR alpha and TR beta. Our previous findings using pharmacological and other approaches supported that TR alpha plays a pivotal role in mediating T-3 actions on neural cell proliferation in Xenopus tadpole brain. Here we used Xenopus tropicalis (X. tropicalis) tadpoles with an inactivating mutation in the gene that encodes TR alpha to investigate roles for TR alpha in mitosis and gene regulation in tadpole brain. Gross morphological analysis showed that mutant tadpoles had proportionally smaller brains, corrected for body size, compared with wildtype, both during prometamorphosis and at the completion of metamorphosis. This was reflected in a large reduction in phosphorylated histone 3 (pH3; a mitosis marker) immunoreactive (ir) nuclei in prometamorphic tadpole brain, when T-3-dependent cell proliferation is maximal. Treatment of wild type premetamorphic tadpoles with T-3 for 48 h induced gross morphological changes in the brain, and strongly increased pH3-ir, but had no effect in mutant tadpoles. Thyroid hormone induction of the direct TR target genes thrb, klf9, and thibz was dysregulated in mutant tadpoles. Analysis of gene expression by RNA sequencing in the brain of premetamorphic tadpoles treated with or without T-3 for 16 h showed that the TR alpha accounts for 95% of the gene regulation responses to T-3.